Nonvolatile memory and manufacturing method thereof

ABSTRACT

Memory elements, switching elements, and peripheral circuits to constitute a nonvolatile memory are integrally formed on a substrate by using TFTs. Since semiconductor active layers of memory element TFTs are thinner than those of other TFTs, impact ionization easily occurs in channel regions of the memory element TFTs. This enables low-voltage write/erase operations to be performed on the memory elements, and hence the memory elements are less prone to deteriorate. Therefore, a nonvolatile memory capable of miniaturization can be provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nonvolatile memory using thin-filmtransistors that are formed by using the SOI (silicon on insulator)technology and, particularly, to an EEPROM (electrically erasable andprogrammable read-only memory) that is formed on an insulative substrateso as to be integral with its peripheral circuits such as a drivercircuit. The term “silicon” as used above means a silicon single crystalor a silicon semiconductor that is substantially a single crystal.

2. Description of the Related Art

In recent years, the miniaturization of semiconductor devices haverequired memories that has high performance and large storage capacityand is small in size. Currently, among various storage devices forsemiconductor devices, the magnetic disk and the semiconductornonvolatile memory manufactured by using bulk silicon are used mostfrequently.

Although the magnetic disk is one of storage devices having the largeststorage capacity among those used for semiconductor devices, it hasdisadvantages of difficulty in miniaturization and slow write and readspeeds.

On the other hand, although at present the semiconductor nonvolatilememory is lower in storage capacity than the magnetic disk, its read andwrite speeds are tens of times higher than the magnetic disk. Further,semiconductor nonvolatile memory products having sufficient performancealso in the number of allowable rewrite operations and data holding timehave been developed recently. This has caused a tendency of using asemiconductor memory as a replacement of a magnetic disk.

However, conventionally, the semiconductor nonvolatile memory ismanufactured by using bulk silicon and accommodated in a package.Therefore, when such a semiconductor nonvolatile memory is mounted on asemiconductor device, the number of manufacturing steps increases and alarge-sized package is an obstacle to miniaturization of thesemiconductor device.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstances,and an object of the invention is therefore to provide a nonvolatilememory which can be formed so as to be integral with other parts of asemiconductor device and allows its miniaturization.

To attain the above object, according to a first aspect of theinvention, there is provided a nonvolatile memory in which memory cellseach including a memory TFT and a switching TFT are arranged in matrixform, wherein the memory TFT comprises a semiconductor active layerformed on an insulative substrate, a gate insulating film, a floatinggate electrode, an anodic oxide film formed by anodizing the floatinggate electrode, and a control gate electrode; the switching TFTcomprises a semiconductor active layer formed on the insulativesubstrate, a gate insulating film, and a gate electrode; and the memoryTFT and the switching TFT are integrally formed on the insulativesubstrate, and the semiconductor active layer of the memory TFT isthinner than that of the switching TFT.

The semiconductor active layers of the memory TFT and the switching TFTmay have a thickness of 150 nm or less.

The semiconductor active layers of the memory TFT and the switching TFTmay have thicknesses of 1-50 nm and 40-100 nm, respectively.

The semiconductor active layer of the memory TFT may have a thickness of10-40 nm.

The semiconductor active layer of the memory TFT may have such thicknessthat impact ionization occurs more easily than that of the switchingTFT.

A tunnel current flowing through the semiconductor active layer of thememory TFT may be two times or more larger than that flowing through thesemiconductor active layer of the switching TFT.

To attain the above object, according to a second aspect of theinvention, there is provided a nonvolatile memory in which memory cellseach including a memory TFT and a switching TFT are arranged in matrixform, wherein the memory TFT comprises a control gate electrode formedon an insulative substrate, a first insulating film, a floating gateelectrode, a second insulating film, and a semiconductor active layer;the switching TFT comprises a gate electrode formed on the insulativesubstrate, a first insulating film, and a semiconductor active layer;and the memory TFT and the switching TFT are integrally formed on theinsulative substrate, and the semiconductor active layer of the memoryTFT is thinner than that of the switching TFT.

The semiconductor active layers of the memory TFT and the switching TFTmay have a thickness of 150 nm or less.

The semiconductor active layers of the memory TFT and the switching TFTmay have thicknesses of 1-50 nm and 40-100 nm, respectively.

The semiconductor active layer of the memory TFT may have a thickness of10-40 nm.

The semiconductor active layer of the memory TFT may have such thicknessthat impact ionization occurs more easily than that of the switchingTFT.

A tunnel current flowing through the semiconductor active layer of thememory TFT may be two times or more larger than that flowing through thesemiconductor active layer of the switching TFT.

To attain the above object, according to a third aspect of theinvention, there is provided a manufacturing method of a nonvolatilememory, comprising the steps of forming first and second amorphoussilicon films having different thicknesses; crystallizing the first andsecond amorphous silicon films into first and second crystalline siliconfilms having first and second thicknesses, respectively, the firstthickness being smaller than the second thickness; and forming a memoryTFT and a switching TFT on the first and second crystalline siliconfilms, respectively.

The first and second thicknesses may be 150 nm or less.

The first and second thicknesses may be 1-50 nm and 40-100 nm,respectively.

The first thickness may be 10-40 nm.

The first and second thicknesses may be so set that impact ionizationoccurs more easily in the first crystalline silicon film than in thesecond crystalline silicon film.

A tunnel current flowing through the semiconductor active layer of thememory TFT may be two times or more larger than that flowing through thesecond crystalline silicon film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a nonvolatile memory according to a firstembodiment of the present invention;

FIG. 2 is a sectional view of a memory element and a switching elementconstituting the nonvolatile memory of FIG. 1;

FIGS. 3A-3D, 4A-4D, 5A-5D, 6A-6D, and 7A-7B show a manufacturing processof a nonvolatile memory according to the first embodiment;

FIGS. 8A-8C are a top view, a sectional view, and a circuit diagram ofmemory elements and switching elements constituting the nonvolatilememory according to the first embodiment;

FIGS. 9A-9, 10A-10D, and 11A-11B show a manufacturing process of anonvolatile memory according to a fourth embodiment of the invention;

FIGS. 12A-12D show examples of display devices according to a fifthembodiment of the invention to which a nonvolatile memory of theinvention is used;

FIGS. 13A and 13B are TEM photographs of crystal grains of semiconductorthin films;

FIGS. 14A-14C are photographs and a schematic diagram of electrondiffraction patterns of semiconductor thin films;

FIGS. 15A and 15B are TEM photographs of crystal grains of semiconductorthin films;

FIGS. 16A and 16B are TEM photographs showing dark field images ofsemiconductor thin films;

FIG. 17 is a graph showing a result of an X-ray diffraction measurementon a semiconductor thin film;

FIG. 18 is a TEM photograph showing a dark field image of asemiconductor thin film;

FIGS. 19A-19C are TEM photographs and an electron beam diffractionpattern of a grain boundary of a semiconductor thin film;

FIGS. 20A-20C are TEM photographs and an electron beam diffractionpattern of another grain boundary of the semiconductor thin film; and

FIGS. 21A-21C are TEM photographs and an electron beam diffractionpattern of still another grain boundary of the semiconductor thin film.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

This embodiment is directed to a nonvolatile memory formed on aninsulative substrate, particularly an EEPROM. An EEPROM according tothis embodiment is formed on an insulative substrate so as to beintegral with its peripheral circuits such as a driver circuit.

FIG. 1 is a circuit diagram of a 4-kilobit EEPROM according to thisembodiment. As shown in FIG. 1, the 4-kilobit EEPROM according to thisembodiment is composed of a plurality of electrically erasable memoryelements Tr1, a plurality of switching elements Tr2, X and Y addressdecoders 101 and 102, and other peripheral circuits 103 and 104. Theperipheral circuits 103 and 104 include an address buffer circuit and acontrol logic circuit and are provided when necessary. Each memoryelement (storage element) Tr1, which stores bit information in FIG. 1,is a p-channel nonvolatile memory element having a floating gate. Eachswitching element Tr2 is an n-channel switching element.

The drain electrodes of two TFTs (TR1 and Tr2) are connected to eachother in series and this series circuit constitutes a 1-bit memory cell.In this embodiment, memory cells each having such a structure arearranged in a matrix of 64×64 (vertical/horizontal). Since each memorycell can store 1-bit information, the EEPROM of this embodiment has astorage capacity of 4,096 bits (about 4 kilobits). Although thisembodiment is directed to the 4,096-bit EEPROM, the invention can beutilized in constructing an EEPROM of any storage capacity.

Both ends of memory cells arranged on each column are connected tosignal lines A0 and B0 to A63 and B63. The gate electrodes of memorycells arranged on each row are connected to signal lines C0 and D0 toC63 to D63. In this embodiment, the respective memory cells constitutingthe 4-kilobit EEPROM are given symbols (0, 0), (1, 0), . . . , and (63,63) as shown in FIG. 1.

The signal lines A0/B0 to A63 and B63 are connected to the Y addressdecoder 102 and the signal lines C0,D0 to C63 to D63 are connected tothe X address decoder 101. A particular memory cell is designated by theX address decoder 101 and the Y address decoder 102 and data is writtento it or read or erased from it.

The configuration of each memory cell according to this embodiment willbe described below with reference to FIG. 2, which is a sectional viewof a memory cell according to this embodiment. In FIG. 2, the left-handelement is a memory element Tr1 and the right-hand element is aswitching element Tr2. A semiconductor active layer 202 of the memoryelement Tr1 includes a source region 203, a drain region 204, and achannel region 205. A semiconductor active layer 206 of the switchingelement Tr2 includes a source region 207, a drain region 208,low-concentration impurity regions 209, and a channel region 210.Reference numerals 211 and 212 denote gate insulating films; 213, afloating gate electrode; 214 and 218, anodic oxide films; 215, a controlgate electrode; 216 and 220, source electrodes; 219, a drain electrode;and 221, an interlayer insulating film.

As shown in FIG. 2, the thickness d1 of the semiconductor active layer202 of the memory element Tr1 is different from the thickness d2 of thesemiconductor active layer 206 of the switching element Tr2, that is,d1<d2. This structure facilitates impact ionization in the semiconductoractive layer 202 of the memory to element Tr1 more than Tr2, which inturn facilitates charge injection into the floating gate electrode 213of the memory element Tr1. Further, it is preferable that tunnel currentflowing through the semiconductor active layer of the memory element betwo times or more larger than that flowing through the semiconductoractive layer of the switching element. This allows writing and erasureto be performed on the memory element at a low voltage and therebydecrease the rate of deterioration of the memory element with respect tothe number of write operations on the memory element.

Semiconductor active layers of TFTs constituting the X and Y addressdecoders 101 and 102 and TFTs constituting other peripheral circuitshave basically the same thickness as the semiconductor active layer ofthe switching TFT Tr2.

The operation of the EEPROM according to this embodiment will bedescribed below by taking the memory cell (1, 1) as an example.

First, to write data to the memory cell (1, 1), a voltage −5 V isapplied to the signal line A1 and a voltage 5 V is applied to the signalline D1. If the signal line B1 is grounded and a high voltage of about20 V is applied to the signal line C1 in this state, carriers (in thiscase, holes) moving through the channel region of the memory element Tr1are accelerated and weak avalanche breakdown or impact ionization occursto cause a large number of high-energy hot carriers (electrons). Thesecarriers are injected into the gate insulating film 211 and trapped inthe floating gate electrode 213. In this manner, charge is accumulatedin the floating gate electrode 213 of the memory element Tr1 and itsthreshold voltage is thereby varied.

Next, to read data from the memory cell (1, 1), 0 V is applied to thesignal line C1 and 5 V is applied to the signal line D1. If the signalline B1 is grounded in this state, the threshold voltage of the memoryelement Tr1 is varied depending on whether charge is accumulated in thefloating gate electrode 213, and a stored signal is read out from thesignal line A1.

Next, to erase data stored in the memory cell (1, 1), 5 V is applied tothe signal line D1 and the signal line B1 is grounded. If a voltage ofabout −20 V is applied to the signal line C1 in this state, electronstrapped in the floating gate electrode 213 are injected into the drainregion 204, whereby stored data is erased.

Table 1 summarizes the above operations.

TABLE 1 A1 (V) B1 (V) C1 (V) D1 (V) Writing 0/−5 GND 20 −5 Reading — GND 0 −5 Erasure — GND −20   −5

The voltages to be applied to the memory element depend on the thicknessof the semiconductor active layer of the memory element, the capacitancebetween the control gate electrode and the floating gate electrode, andother factors. Therefore, the operation voltages of the memory elementare not limited to the above values.

The number of allowable rewrite operations and the information holdingtime are important characteristics of the EEPROM. To increase the numberof allowable rewrite operations, the voltage applied to the control gateelectrode of the memory element needs to be small. Since thesemiconductor active layer of the memory element of this embodiment isthinner than the semiconductor active layers of the switching TFT andthe TFTs constituting the address decoders 101 and 102, impactionization easily occurs there and hence the voltage to be applied tothe control gate electrode 215 can be set low.

In this embodiment, when data is written to or erased from the memoryelement Tr1, pulses lower than 20 V may be applied to the control gateelectrode 215 of the memory element Tr1 a plurality of times rather thana voltage 20 V is applied thereto once. This prevents deterioration ofthe memory element Tr1.

The TFT to constitute the EEPROM according to this embodiment isrequired to be superior in mobility, threshold voltage, and othercharacteristics. Therefore, TFTs having an amorphous siliconsemiconductor active layer, which are commonly used conventionally, areinsufficient in performance. A method for manufacturing TFTs exhibitingsuch superior characteristics will be described below. The followingmanufacturing method enables manufacture of TFTs having superiorcharacteristics and thereby realizes the EEPROM according to thisembodiment.

A manufacturing method of the EEPROM according to this embodiment willbe described below with reference to FIGS. 3A-3D to 7A-7B. In thesefigures, a memory element and a switching element that constitute amemory cell and two TFTs constituting a CMOS circuit that is typical ofcircuits to constitute the address decoders 101 and 102 and otherperipheral circuits are used as examples of the TFTs that constitute theEEPROM according to this embodiment.

It will be understood that according to the following manufacturingmethod of a nonvolatile memory the nonvolatile memory of this embodimentcan be formed integrally with any semiconductor device that can bemanufactured by the thin-film technology.

As shown in FIG. 3A, first a quartz substrate 301 is prepared as asubstrate having an insulative surface. Alternatively, a siliconsubstrate formed with a thermal oxide film may be used. As a furtheralternative, an amorphous silicon film may be formed on a quartzsubstrate and then thermally oxidized in its entirety into an insulatingfilm. It is also possible to use a quartz substrate or a ceramicsubstrate formed with a silicon nitride film as an insulating film.

Then, a 25-nm-thick amorphous silicon film 302 is formed (see FIG. 3A).In this embodiment, the film is formed by low-pressure CVD under thefollowing conditions:

film forming temperature: 465° C. film forming pressure: 0.5 Torr; andfilm forming gases: He (helium) 300 sccm Si₂H₆ (disilane) 250 sccm.

Thereafter, a resist film is formed and then patterned into a mask 304(see FIG. 3B). Subsequently, the amorphous silicon film 303 is etched toform an amorphous silicon film 304 that occupies part of the surface ofthe substrate 301 (see FIG. 3C). The amorphous silicon film 303 may beetched by either dry etching or wet etching. In the case of dry etching,CF₄+O₂ may be used. In the case of wet etching, hydrofluoric acid plusnitric acid may be used.

Then, a 50-nm-thick amorphous silicon film is again formed by theabove-described method, to form amorphous silicon films 305 and 306 asshown in FIG. 3D. In this embodiment, adjustments are so made that theamorphous silicon films 305 and 306 will have final thicknesses (i.e.,thicknesses after thickness reduction due to thermal oxidation) of 50 nmand 75 nm, respectively.

It is desirable that the surfaces of the amorphous silicon film 304 andthe quartz substrate 301 be cleared before the second formation of anamorphous silicon film.

The amorphous silicon films 305 and 306 may be formed by another method.For example, a 75-nm-thick amorphous silicon film is formed over theentire surface by the above-described method, a mask is formed so as tocover part of it, and the amorphous silicon film is etched in theabove-described method so as to be reduced in thickness partially.

The amorphous silicon film 305 will become a semiconductor active layerof a memory element and the amorphous silicon film 306 will becomesemiconductor active layers of a switching element and a peripheral CMOScircuit or the like.

If the final thicknesses of the semiconductor active layers are 150 nmor more, particularly 200 nm or more, the degree of impact ionizationthat is characteristic of the SOI structure would be extremely low, thatis, as low as in a nonvolatile memory using bulk silicon. Therefore, inthe invention, it is preferable that the final thicknesses of both kindsof semiconductor active layers be 150 nm or less (preferably 100 nm orless).

Although in this embodiment the final thicknesses of the amorphoussilicon film 305 for a memory element and the amorphous silicon film 306for a switching element and a peripheral CMOS circuit or the like areset to 50 nm and 75 nm, respectively, in the invention they are notlimited to those values. The thicknesses of the amorphous silicon films305 and 306 may preferably be set in ranges of 1-50 nm (even preferably10-40 nm) and 40-100 nm, respectively.

In forming the amorphous silicon films, it is important to thoroughlymanage the concentrations of impurities in the films. In thisembodiment, management is so made that the concentrations of C (carbon)and N (nitrogen) in the amorphous silicon films 305 and 306 become lowerthan 5×10¹⁸ atoms/cm³ (typically 5×10¹⁷ atoms/cm³ or less, preferably2×10¹⁷ atoms/cm³ or less) and the concentration of O (oxygen) becomelower than 1.5×10¹⁹ atoms/cm³ (typically 1×10¹⁸ atoms/cm³ or less,preferably 5×10¹⁷ atoms/cm³ or less). (C, N, and O arecrystallization-obstructing impurities.) This is because if eachimpurity exists at a concentration higher than the above value, it willadversely affect later crystallization to thereby degrade the quality ofcrystallized films. In this specification, the concentration of animpurity element in a film, like the above concentrations, is defined asthe minimum value of SIMS (secondary ion mass spectroscopy) measurementvalues.

For the above concentration management, it is desirable that alow-pressure CVD furnace used in this embodiment be subjected to drycleaning on a regular basis so that a film forming chamber be keptclean. The dry cleaning may be performed in such a manner that a ClF₃(chlorine fluoride) gas is introduced at 100-300 sccm into the furnacethat is heated to 200°-400° C. so that the film forming chamber iscleaned by fluorine that is generated through thermal decomposition.

According to the knowledge of the inventors, attached substances (mainlymade of silicon) of about 2 μm in thickness can be removed completely in4 hours when the furnace inside temperature and the flow rate of a ClF₃gas are set to 300° C. and 300 sccm, respectively.

The concentration of hydrogen in the amorphous silicon films 305 and 306is also a very important parameter. Films having higher crystallinitycan be obtained by making the hydrogen content lower. To this end, it ispreferably to form the amorphous silicon films 305 and 306 bylow-pressure CVD. Plasma CVD can also be used if the film formingconditions are optimized.

Then, the amorphous silicon films 305 and 306 are crystallized by usinga technique disclosed in Japanese Patent Laid-Open No. Hei. 7-130652 ofthe present inventors. Although either of the techniques disclosed inthe first and second embodiments of this publication may be used, in theinvention it is preferable to use the technique of the second embodiment(disclosed in more detail in Japanese Patent Laid-Open No. Hei.8-78329).

In the technique of the publication No. Hei. 8-78329, first maskinsulating films 307-309 for selecting regions where to add a catalystelement are formed. Thereafter, a solution containing nickel (Ni) as acatalyst element for accelerating crystallization of the amorphoussilicon films 305 and 306 is applied by spin coating, to form aNi-containing layer 310 (see FIG. 4A).

Other examples of the catalyst element are cobalt (Co), iron (Fe),palladium (Pd), platinum (Pt), copper (Cu), gold (Au), germanium (Ge),lead (Pb), indium (In) and the like.

The catalyst element adding method is not limited to spin coating andmay be ion implantation or plasma doping using a resist mask. The use ofion implantation or plasma doping is effective in constructing aminiaturized circuit because it is easy to reduce the areas occupied bycatalyst-element-added regions and to control the growth distance of alateral growth region.

After the completion of the catalyst element adding step, hydrogenremoval is performed at 450° C. for about 1 hour. Subsequently, theamorphous silicon films 305 and 306 are crystallized by performing aheat treatment at 500°-700° C. (typically 550°-650° C.) for 4-24 hoursin an inert atmosphere, a hydrogen atmosphere, or an oxygen atmosphere.In this embodiment, a heat treatment is to performed at 570° C. for 14hours in a nitrogen atmosphere.

In this step, the crystallization of the amorphous silicon films 305 and306 proceeds with priority from nuclei that are generated in regains 311and 312 where nickel is added, whereby crystal regions 313-315 areformed that have grown approximately parallel with the surface of thesubstrate 301 (see FIG. 4B). The inventors call the crystal regions313-315 lateral growth regions. The lateral growth region has anadvantage of superior overall crystallinity because it is a collectionof individual crystals that are relatively similar in crystallinity.

After the completion of the heat treatment for crystallization, the maskinsulating films 307-309 are removed. Then, patterning is performed toform island-like semiconductor layers (active layers) 316-319 made fromonly the lateral growth regions (see FIG. 4C).

Then, after a channel forming region of the island-like semiconductoractive layer 316 and the other island-like semiconductor active layers317-319 are covered with resist masks 320 and 321, impurity ions forimparting p-type conductivity are added. Although B (boron) is used asthe impurity element, In (indium) may also be used. The accelerationvoltage at the time of the impurity addition is set to about 80 kV.

As a result, a source region 322, a drain region 323, and a channelforming region 324 are formed in the island-like semiconductor activelayer 316. Since the island-like semiconductor active layers 317-319 arecovered with the resist mask 321, the impurity is not added thereto (seeFIG. 4D).

Then, after the resist masks 320 and 321 are removed, a gate insulatingfilm is formed that is an insulating film 325 containing silicon (seeFIG. 5A). The thickness of the gate insulating film 325 may be adjustedin a range of 10-250 nm (thickness increase in a later thermal oxidationstep is taken into account). The thickness of the gate insulating film325 on the island-like semiconductor active layer 316 for a memoryelement may be set to 10-50 nm and the other portion may be set to50-250 nm. The gate insulating film 325 may be made of SiO₂, SiON, SiN,or the like. The film forming method may be a known vapor-phase method(plasma CVD, sputtering, or the like).

Then, to remove or reduce the concentration of the catalyst element(nickel), a heat treatment (catalyst element gettering process) isperformed as shown in FIG. 5A. In this heat treatment, to utilize ametal element gettering effect of a halogen element, a halogen elementis added to a processing atmosphere.

To fully utilize the gettering effect of the halogen element, it ispreferable that the heat treatment be performed at a temperature higherthan 700° C. If the temperature is lower than 700° C., the halogenelement in the processing atmosphere is hard to decompose and thegettering effect may not be obtained. Therefore, it is preferable thatthe heat treatment temperature and the processing time be set to800°-1,000° C. (typically 950° C.) and 0.1-6 hours (typically 0.5-1hour). In the heat treatment, it is necessary to take a proper measureto prevent impurities in the source and drain regions 322 and 323 fromdiffusing into the channel region 324.

In a typical embodiment, a heat treatment is performed at 950° C. for 30minutes in an oxygen atmosphere containing a hydrogen chloride (HCl) gasat 0.5-10 vol % (in this embodiment, 3 vol %). A HCl content higher thanthis range is not preferable because asperities whose height is on theorder of the thickness are formed on the surface of the active layers316-319.

Other than HCl the compound containing a halogen element may be one or aplurality of compounds selected from HF, NF₃, HBr, Cl₂, ClF₃, BCl₃, F₂,Br₂ and the like.

In this step, nickel in the active layers 316-319 are removed in such amanner that it is gettered by the action of chlorine and desorbed intothe air in the form of nickel chloride which is volatile. As a result ofthis step, the nickel concentration of the active layers 316-319 isreduced to 5×10¹⁷ atoms/cm³ or less (typically 2×10¹⁷ atoms/cm³ orless). According to the experience of the present inventors, no adverseeffects occur in the TFT characteristics as long as the nickelconcentration is 1×10¹⁸ atoms/cm³ or less (preferably 5×10¹⁷ atoms/cm³or less).

The above gettering process is also effective for metal elements otherthan nickel. Metal elements that may be mixed into the silicon films aremainly elements that constitute the film forming chamber (typically,aluminum, iron, and chromium). The concentrations of those metalelements can also be reduced to 5×10¹⁷ atoms/cm³ or less (preferably2×10¹⁷ atoms/cm³ or less) by executing the above gettering process.

After the above gettering process, the halogen element that was used inthe gettering process remains in the active layers 316-319 at aconcentration of 1×10¹⁶ to 1×10²⁰ atoms/cm³.

In the above heat treatment, a thermal oxidation reaction proceeds atthe interfaces between the gate insulating film 325 and the activelayers 316-319, whereby the thickness of the gate insulating film 325 isincreased by the thickness of a thermal oxidation film. By forming athermal oxidation film in this manner, a semiconductor/insulating filminterface where the concentration of interface states is very low can beobtained. Another advantage can be obtained that improper formation of athermal oxidation film (edge thinning) at the end portions of the activelayers 316-319 can be prevented.

It is effective to improve the film quality of the gate insulating film325 by performing a heat treatment at 950° C. for about 1 hour in anitrogen atmosphere after the above heat treatment in ahalogen-containing atmosphere.

Thereafter, a metal film (not shown) having aluminum as the maincomponent is formed and then patterned into starting members 326-329 forlater gate electrodes (see FIG. 5B). In this embodiment, an aluminumfilm containing scandium at 2 wt % is used. Alternatively, a tantalumfilm, a conductive silicon film, or the like may be used.

In this state, a technique disclosed in Japanese Patent Laid-Open No.Hei. 7-135318 of the present inventors is utilized. This publicationdiscloses a technique of forming source and drain regions andlow-concentration impurity regions in a selfaligned manner by using anoxide film formed by anodization. This technique will be brieflydescribed below.

First, anodization is performed in a 3%-aqueous solution of oxalic acidin a state that the resist mask (not shown) that was used for thepatterning of the aluminum film is left, whereby porous anodic oxidefilms 330-337 are formed. Since low-concentration impurity regions willbe formed at a length that is equal to the thickness of the porousanodic oxide films 330-337, the thickness is controlled in considerationof a desired length of low-concentration impurity regions.

After the resist mask (not shown) is removed, anodization is performedin an electrolyte obtained by mixing tartaric acid at 3% with anethylene glycol solution. As a result, dense, non-porous anodic oxidefilms 338-341 are formed. The thickness may be set to 70-120 nm.

Aluminum films 342-345 that remain after the two anodization steps willsubstantially function as gate electrodes (see FIG. 5C). The aluminumfilm 342 will become a floating gate electrode of a memory element.

Then, the gate insulating film 325 is etched by dry etching by using thegate electrodes 342-345 and the porous anodic oxide films 330-337 asmasks, whereby the gate insulating film 325 is pattered into gateinsulating films 346-349 (see FIG. 5D).

Then, the porous anodic oxide films 330-337 are removed (see FIG. 6A).The end portions of the thus-formed gate insulating films 346-349project by a length that is equal to the thickness of the porous anodicoxide films 330-337.

Thereafter, the gate electrode 342 is divided and a floating gateelectrode 342′ is formed (see FIG. 6B).

Then, a step of adding an impurity element for imparting oneconductivity type is executed. The impurity element may be P(phosphorus) or As (arsenic) for an N type and B (boron) or In (indium)for a p type.

First, to add an impurity for an n-type TFT, resist masks 350 and 351are formed. In this embodiment, the impurity is added in two steps. Thefirst impurity addition step (in this embodiment, P (phosphorus) isused) is executed with a high acceleration voltage of about 80 kV,whereby n⁻ regions 356, 357 are formed. Adjustments are so made that then⁻ regions have a P concentration of 1×10¹⁷ to 1×10¹⁹ atoms/cm³.

Subsequently, the second impurity addition step is performed with a lowacceleration voltage of about 10 kV, whereby n⁺ regions 352-355 areformed. In this step, because of the low acceleration voltage, the gateinsulating films 347 and 349 function as masks. Adjustments are so madethat the n⁺ regions 352-355 have a sheet resistance of 500Ω or less(preferably 300Ω or less).

As a result, source and drain regions 352-355, low-concentrationimpurity regions 356 and 357, and channel regions 358 and 359 of n-typeTFTs are formed.

Then, after the n-type TFTs are covered with resist masks 360 and 361,impurity ions for imparting p-type conductivity (in this embodiment, B(boron) ions) are added, whereby p⁻ region 364 and p⁺ regions 362, 363are formed. Adjustments are so made that the p⁻ region 364 have a Bconcentration of 1×10¹⁷ atoms/cm³ or more (preferably 1×10¹⁸ atoms/cm³or more). Other than B, Ga, In, or the like may be used.

As a result, source and drain regions 362 and 363, low-concentrationimpurity regions 364, and a channel forming region 367 of a p-type TFTare formed (see FIG. 6D).

Since the low-concentration impurity regions are formed in the switchingTFT and the TFTs for a peripheral circuit as described above, impactionization is hard to occur even though the semiconductor active layersare thin.

After the active layers have been completed in the above manner, theimpurity elements are activated by a combination of furnace annealing,laser annealing, lamp annealing, etc. Damage of the active layers thatwas caused in the impurity addition steps is repaired at the same time.

Having a small number of unmatched bonds, the channel forming regions ofthe TFTs according to this embodiment can substantially be regarded assingle crystals.

Then, a 500-nm-thick interlayer insulating film 368 is formed. Theinterlayer insulating film 368 may be a silicon oxide film, a siliconnitride film, a silicon oxynitride film, an organic resin film, or alaminated film thereof.

Subsequently, after contact holes are formed, source and drainelectrodes 369-374 and a control gate electrode 375 of the memoryelement is formed so as to be connected to the top surface of the anodicoxide film 338 (see FIG. 7B).

Finally, the entire device is subjected to hydrogenation by heating theentire substrate at 350° C. for 1-2 hours in a hydrogen atmosphere,whereby dangling bonds in the films are terminated. As a result of theabove steps, TFTs having structures of FIG. 7B are manufactured.

Knowledge Relating to Impurities contained in Active Layers

Active layers (semiconductor thin films) formed according to thisembodiment has a feature that C (carbon), N (nitrogen), and O (oxygen),which are crystallization-obstructing impurities, do not exist at all orsubstantially no such elements exist there. This is attained by thoroughmanagement of impurities (contaminants).

In this embodiment, because the introduction of C, N, and O is avoidedin a thoroughgoing manner in forming amorphous silicon films, theconcentrations of C and N finally remaining in semiconductor films arenecessarily reduced to at most less than 5×10¹⁸ atoms/cm³ (typically5×10¹⁷ atoms/cm³ or less, preferably 2×10¹⁷ atoms/cm³ or less) and theconcentration of O is necessarily reduced to at most less than 1.5×10¹⁹atoms/cm³ (typically 1×10¹⁸ atoms/cm³ or less, preferably 5×10¹⁷atoms/cm³ or less).

Since a semiconductor film of pure silicon has a silicon concentrationof about 5×10²² atoms/cm³, an impurity element concentration of, say,5×10¹⁸ atoms/cm³ corresponds to about 0.01 atomic %.

To obtain superior crystallinity, it is desirable that theconcentrations of C, N, and O in a final semiconductor film be less thanthe lower detection limit of the SIMS analysis, and it is more desirableto completely eliminate those impurity elements.

SIMS analyses conducted by the inventors have revealed that if anamorphous silicon film in which the C, N, and O concentrations arewithin the above ranges is used as a starting film, the C, N, and Oconcentrations of an active layer of a completed TFT also fall withinthe above ranges.

FIG. 8A is a circuit arrangement diagram of the nonvolatile memoryaccording to this embodiment. FIG. 8B is a sectional view taken alongline A-A′ in FIG. 8A and FIG. 8C is an equivalent circuit diagram ofFIG. 8A.

In FIG. 8A, reference numerals 801-808 denote semiconductor activelayers of TFTs Tr1-Tr8. First wiring layer lines 809-812 are used asgate electrodes and gate signal lines of Tr2, Tr4, Tr6, and Tr8 and gatesignal lines of Tr1, Tr3, Tr5, and TrY. Floating gate electrodes 813-816of Tr1, Tr3, Tr5, and Tr7 are formed at the same time as the firstwiring layers 809-812 and rendered in a floating state after patterning.Second wiring layer lines 817-828 are used as signal lines connected tosource and drain regions of the respective TFTs Tr1-Tr8 and control gateelectrodes of Tr1, Tr3, Tr5, and Tr7. In FIG. 8A, solid portionsindicate contacts to underlying lines or semiconductor layers. Further,in FIG. 8A, the lines hatched in the same pattern belong to the samewiring layer.

In the nonvolatile memory according to this embodiment, since thesemiconductor active layers of the memory elements are made thinner thanthose of the switching elements and elements constituting peripheralcircuits, impact ionization more easily occurs in the memory elementsand hence writing and erasure on the memory elements can be performed atlow voltages. This reduces the rate of deterioration of the memoryelements with respect to the number of write/erase operations. This isan innovative solution to a problem that the gate insulating film of aconventional EEPROM using bulk silicon is prone to deteriorate becauseit is relatively thin, as well as a solution to a problem that in aconventional EEPROM using bulk silicon, because of a thin gateinsulating film, carriers once accumulated in the floating gate flow outdue to a temperature increase.

Embodiment 2

According to this embodiment, first, an inexpensive, low-grade quartzsubstrate is prepared. Then, the quartz substrate is polished into anideal state (average height difference of asperities: within 5 nm,typically within 3 nm, preferably within 2 nm) by CMP (chemicalmechanical polishing) or the like.

In this manner, by subjecting it to polishing, even an inexpensivequartz substrate can be used as an insulative substrate that is superiorin flatness. Since a quartz substrate provides a very dense underlyingmember, the interface between the underlying member and a semiconductorthin film is made highly stable. With an additional advantage that thereis almost no contaminative influence from the substrate, this embodimentis very useful.

Embodiment 3

In the first embodiment, a halogen element is used in the step ofgettering the catalyst element for accelerating crystallization ofsilicon. In the invention, the element of phosphorus can also be used inthe catalyst element gettering step (this embodiment). The other stepsare the same as in the first embodiment.

Where the element of phosphorus is used, phosphorus may be added toregions other than regions to become active layers and a heat treatmentmay be performed at 400°-1,050° C. (preferably 600°-750° C.) for 1minute to 20 hours (typically 30 minutes to 3 hours). As a result of theheat treatment, the catalyst element is gettered in the regions wherephosphorus is added, whereby the concentration of the catalyst elementin the regions to become active layers is reduced to 5×10¹⁷ atoms/cm³ orless.

After the completion of the gettering step, active layers are formed byusing the regions other than the regions where phosphorus is added.Thereafter, the same steps as in the first embodiment are executed. As aresult, a semiconductor device having the same features as in the firstembodiment is obtained.

Naturally, a heat treatment may also be performed in an atmospherecontaining a halogen element in forming a thermal oxidation film tobecome a gate insulating film. In this case, a multiplier effect of thegettering effect of the element of phosphorus and that of the halogenelement can be obtained.

Embodiment 4

This embodiment is directed to a case of constructing an EEPROM by usinginverted staggered structure TFTs. This embodiment will be describedwith reference to FIGS. 9A-9D to 11A-11B. Although in FIGS. 9A-9D to11A-11B attention is paid to only one memory cell (a memory element anda switching element), address decoders, peripheral circuits, and thelike are also formed simultaneously. Actually, as described in the firstembodiment in connection with FIG. 1, an EEPROM is constructed by aplurality of memory cells in matrix form, address decoders, andperipheral circuits.

First, as shown in FIG. 9A, a silicon oxide film as an undercoat film902 is formed on a glass substrate 901 and gate electrodes 903 and 904are formed thereon. The gate electrode 903 will become a control gateelectrode of a memory element and the gate electrode 904 will become agate electrode of a switching element. Although this embodiment employschromium films of 200-400 nm in thickness as the gate electrodes 903 and904, they may be made of an aluminum alloy, tantalum, tungsten,molybdenum, or the like, or may be a conductive silicon film or thelike.

Then, a gate insulating film 905 of 100-200 nm in thickness is formed soas to cover the gate electrodes 903 and 904. The gate insulating film905 may be a silicon oxide film, a silicon nitride film, or a laminatedfilm of a silicon oxide film and a silicon nitride film. Alternatively,anodic oxide films obtained by anodizing the gate electrodes 903 and 904may be used as gate insulating films.

The portion of the gate insulating film 905 on the side of the memoryelement will define the capacitance between a floating gate electrodeand a control gate electrode that will be formed in the next step.Therefore, the voltage to be applied to the floating gate electrode canbe adjusted by changing the thickness of the memory element side portionof the gate insulating film 905. Therefore, the thickness of the gateinsulating film 905 is not limited to the above range and may be changedpartially.

Then, a floating gate electrode 906 is formed (see FIG. 9B). Althoughthis embodiment employs a chromium film as the floating gate electrode906, it may be made of an aluminum alloy, tantalum, tungsten,molybdenum, or the like, or may be a conductive silicon film or thelike.

Then, an insulating film 907 is formed at a thickness of 1050 nm. Theinsulating film 907 may be a silicon oxide film, a silicon nitride film,or a laminated film of a silicon oxide film and a silicon nitride film.

Thereafter, amorphous silicon films 908 and 909 are formed by the methoddescribed in the first embodiment in connection with FIGS. 3A-3D (seeFIG. 9C). Although in this embodiment the final thicknesses of theamorphous silicon film 908 for the memory element and the amorphoussilicon film 909 for the switching element are set to 50 nm and 75 nm,respectively, in the invention they are not limited to those values. Thethicknesses of the amorphous silicon films 908 and 909 may be set inranges of 1-50 nm (preferably 10-40 nm) and 40-100 nm, respectively.Although not shown in FIG. 9C, the thickness of an amorphous siliconfilm of a TFT for address decoders and peripheral circuits is set thesame as that for the switching element.

Then, the amorphous silicon films 908 and 909 are crystallized byilluminating those with laser light or strong light that is as intenseas laser light (see FIG. 9D). It is preferable to use excimer laserlight as laser light. An excimer laser may be a pulsed laser having KrF,ArF, or XeCl as a light source.

Strong light that is as intense as laser light may be one emitted from ahalogen lamp, a metal halide lamp, or infrared or ultraviolet lamp.

In this embodiment, the entire amorphous silicon films 908 and 909 arecrystallized by scanning the substrate from one end to the other withexcimer laser light that is shaped like a linear beam. At this time, thesweep speed of laser light is set to 1.2 mm/s, the processingtemperature is set to room temperature, the pulse frequency is set to 30Hz, and the laser energy density is set to 300-315 mJ/cm². Crystallinesilicon films are obtained by this step.

In this embodiment, the amorphous silicon films 908 and 909 can also becrystallized by the crystallization method employed in the first orthird embodiment. Conversely, it is understood that the crystallizationmethod of this embodiment can be applied to the amorphous silicon filmsof the first embodiment.

Then, as shown in FIG. 10A, the crystalline silicon films are patternedinto active layers 910 and 911.

Then, an impurity element for imparting one conductivity type is added.First, after the active layer portions to constitute channel regions ofthe memory element, n-type TFTs, and p-type TFTs are covered with resistmasks (not shown), an impurity element for imparting p-type conductivity(boron is used in this embodiment; indium or the like may also be used)is added, whereby p⁻ regions (low-concentration impurity regions; notshown) having a boron concentration of 1×10¹⁷ atoms/cm³ or more(preferably 1×10¹⁸ atoms/cm³ or more) are formed.

Then, after resist masks 912 and 913 are formed (see FIG. 10B), animpurity element for imparting p-type conductivity is added so as toprovide a concentration of about 1×10¹⁸ to 1×10²⁰ atoms/cm³, whereby asource region 914 and a drain region 915 of the p-type TFT are formed.The portion of the active 910 that is covered with the resist mask 912becomes a channel region (see FIG. 10B).

Then, after the resist masks 912 and 913 are removed, resist masks 917and 918 are formed. Subsequently, an impurity for imparting n-typeconductivity (in this embodiment, phosphorus; arsenic or the like mayalso be used) is added to form low-concentration impurity regions 919and 920 of about 1×10¹⁷ to 5×10¹⁸ atoms/cm³ (see FIG. 10C).

Then, after the resist masks 917 and 918 are removed, resist masks 921and 922 are formed. Subsequently, an impurity for imparting n-typeconductivity is again added at a higher concentration (1×10¹⁸ to 1×10²⁰atoms/cm³) than in the step of FIG. 10C, to form source and drainregions 923 and 924 of the n-type TFT. Reference numerals 925 and 926denote low-concentration impurity regions and a channel forming region,respectively (see FIG. 10D).

Then, after the resist masks 921 and 922 are removed, excimer laserlight is applied (laser annealing) to repair damage that was caused inthe ion implanting operations and to activate the added impurities (seeFIG. 11A).

After the completion of the laser annealing, an interlayer insulatingfilm 927 is formed at a thickness of 300-500 nm (see FIG. 11B). Theinterlayer insulating film 927 may be a silicon oxide film, a siliconnitride film, an organic resin film, or a laminated film thereof.

Then, after contact holes are formed through the interlay insulatingfilf 927, metal thin films as source and drain electrodes 928-930 areformed (see FIG. 11B). The metal thin films may be made of aluminum,tantalum, titanium, tungsten, of molybdenum, or may be laminated filmsthereof.

Then, a heat treatment is performed on the entire substrate at 350° C.for about 2 hours in a hydrogen atmosphere, whereby dangling bonds inthe films (particularly the channel forming regions) are terminated byhydrogen. As a result of the above steps, the state of FIG. 11B isobtained.

Embodiment 5

The nonvolatile memories according to the first to fourth embodimentshave various applications. This embodiment is directed to semiconductordevices in which those nonvolatile memories may be used.

Semiconductor devices in which those nonvolatile memories may be usedare a video camera, a still camera, a head-mounted display, a carnavigation apparatus, a personal computer, portable informationterminals (a mobile computer, a cellular telephone, etc.), and the like.FIGS. 12A-12D show examples of those semiconductor devices.

FIG. 12A shows a cellular telephone, which is composed of a main body1201, a voice output section 1202, a voice input section 1203, a displaydevice 1204, manipulation switches 1205, and an antenna 1206. Thenonvolatile memory according to the invention may be formed so as to beintegral with the display device 1204.

FIG. 12B shows a video camera, which is composed of a main body 1301, adisplay device 1302, a sound input section 1303, manipulation switches1304, a battery 1305, and an image receiving section 1306. Thenonvolatile memory according to the invention may be formed so as to beintegral with the display device 1302.

FIG. 12C shows a mobile computer, which is composed of a main body 1401,a camera section 1402, an image receiving section 1403, a manipulationswitch 1404, and a display device 1405. The nonvolatile memory accordingto the invention may be formed so as to be integral with the displaydevice 1405.

FIG. 12D shows a head-mounted display, which is composed of a main body1502, display devices 1502, and a band section 1503. The nonvolatilememory according to the invention may be formed so as to be integralwith the display devices 1502.

Embodiment 6

This embodiment is directed to a case of forming gate electrodes byusing Ta (tantalum) or a Ta alloy in any of the manufacturing methodsdescribed in the first to fourth embodiments.

A gate electrode made of Ta or a Ta alloy can be subjected to thermaloxidation at about 450°-600° C., whereby an oxide film of Ta₂O₃ or thelike having superior film quality can be formed thereon. It has becomeapparent that this oxide film has better film quality than an oxide filmthat is formed on an Al (aluminum) gate electrode as described in thefirst embodiment.

This has been found from the fact that in a J-E characteristic (currentdensity vs. electric field strength characteristic) that is one of thewithstand voltage evaluation items for insulating film oxide films of Taor a Ta alloy exhibit better characteristics than A1 oxide films.

Having relative dielectric constant of about 11.6, Ta₂O₃ can providelarge capacitance between a floating gate electrode and a control gateelectrode. Therefore, a gate electrode made of Ta or a Ta alloy hasanother advantage that charge can easily be injected into a floatinggate electrode than in the case of using an A1 gate electrode.

Further, a gate electrode made of Ta can be anodized as described in theabove embodiments.

Knowledge Relating to CGS

This embodiment is directed to a semiconductor thin film formed by themanufacturing method described in the first embodiment. According to themanufacturing method of the first embodiment, a crystal silicon filmcalled continuous grain boundary crystal silicon (i.e., continuous grainsilicon: CGS) can be obtained by crystallizing an amorphous siliconfilm.

A lateral growth region of a semiconductor thin film obtained by themanufacturing method of the first embodiment has a unique crystalstructure that is a collected body of rod-like or flat-rod-likecrystals. Features of this crystal structure will be described below.

Knowledge relating to crystal structure of active layer

A lateral growth region formed according to the manufacturing process ofthe first embodiment has a crystal structure in which microscopically aplurality of rod-like (or flat-rod-like) crystals are arrangedapproximately parallel with each other with regularity that isassociated with a particular direction. This can easily be confirmed byan observation by a TEM (transmission electron microscope) method.

By a HR-TEM (high-resolution transmission electron microscope) method,the inventors observed, in a detailed manner, a grain boundary of asemiconductor thin film formed by the above-described manufacturingmethod by magnifying it at a magnification factor of eight million (seeFIG. 13A). In this specification, the term “grain boundary” is definedas a grain boundary that is formed at an interface between differentrod-like crystals, unless otherwise specified. Therefore, it isdiscriminated from, for instance, a macroscopic grain boundary as formedby collision between different lateral growth regions.

The HR-TEM method is a technique for evaluating an arrangement of atomsor molecules by utilizing interference between transmission electrons orelastic scattering electrons that are produced by applying an electronbeam to a sample vertically. This technique allows an arrangement stateof crystal lattices to be observed as a lattice fringe. Therefore, abonding state of atoms at a grain boundary can be inferred by observingthe grain boundary.

A TEM photograph (see FIG. 13A) obtained by the inventors clearly showsa state that two different crystal grains (rod-like crystal grains)contact each other at a grain boundary. Although the crystal axes of thetwo crystal grains have a small variation, it has been confirmedelectron beam diffraction that they approximately have a {110}orientation.

Incidentally, in lattice fringe observations by using FEM photographslike the above one, a lattice fringe corresponding to the {111} planewere observed in a lattice fringe of the {110} plane. The term “latticefringe corresponding to the {111} plane” means a lattice fringe in whicha {111} plane appears on a cross section obtained by cutting a crystalgrain along the lattice fringe. A simplified method for checking whichplane a lattice fringe corresponds to is to use the distance betweenlattice fringe stripes

The inventors obtained very interesting knowledge through a detailedobservation of the TEM photograph of FIG. 13A of the semiconductor thinfilm formed by the manufacturing method of the first embodiment. Each ofthe two different crystal grains of the photograph had a lattice fringecorresponding to the {111} plane, and clearly the two lattice fringesran parallel with each other.

Further, irrespective of the presence of the grain boundary, the latticefringes of the two crystal grains were connected to each other so as totraverse the grain boundary. That is, it was found that most of latticefringe stripes that traversed the grain boundary were straight andcontinuous at the grain boundary though they are of the differentcrystal grains. This applies to any grain boundary; 90% or more(typically 95% or more) of all lattice fringe stripes were continuous ata grain boundary.

The above type of crystal structure (more correctly, crystal grainstructure) indicates that the two different crystal grains are joined toeach other with a very high degree of matching at the grain boundary.That is, crystal lattices continuously extend at the grain boundary andtrap states due to crystal defects etc. are less prone to occur. Inother words, it can be said that crystal lattices have continuity at thegrain boundary.

For reference, the inventors performed analyses on a conventionalpolysilicon film (what is called a high-temperature polysilicon film) byelectron beam diffraction and HR-TEM observation (see FIG. 13B). Resultswere such that lattice fringes of two different crystal grains rancompletely independently and there were almost no junctions at the grainboundary where the crystal grains are continuous with each other with ahigh degree of matching. That is, it was found that there were manyportions (part of those are indicated by arrows) at the grain boundarywhere lattice fringe stripes are terminated as well as many crystaldefects. Dangling bonds exist in those portions and act, at a highprobability, as trap states that obstruct movement of carriers.

The inventors call a bonding state of atoms a matched bond for whichlattice fringes are well matched as in the case of a semiconductor thinfilm obtained by the manufacturing method of the first embodiment. Onthe other hand, a bonding state of atoms for which lattice fringes arenot matched well as frequently found in conventional polysilicon filmsis called an unmatched bond (or dangling bond).

A semiconductor thin film used in the nonvolatile memory of theinvention has a very high degree of matching at grain boundaries andhence have very small number of unmatched bonds. The inventors'investigation of arbitrary grain boundaries showed that the ratio ofunmatched bonds to all bonds was 10% or less (preferably 5% or less,even preferably 3% or less). That is, 90% or more (preferably 95% ormore, even preferably 97% or more) of all bonds are matched bonds.

FIG. 14A shows an observation result by electron beam diffraction of alateral growth region formed by the manufacturing method of the firstembodiment. FIG. 14B shows an electron beam diffraction pattern of aconventional polysilicon film (usually called a high-temperaturepolysilicon film) that was observed for comparison.

Since the photographs of FIGS. 14A and 14B were taken with the diameterof an electron beam irradiation spot at 1.35 μm, it can be said thatthey reflect information of a sufficiently macroscopic region ascompared with the case of a lattice fringe.

FIG. 14C is a schematic diagram of an electron beam diffraction patternthat will be obtained when an electron beam is applied vertically to the{110} plane of single crystal silicon. Usually, an orientation of anobservation sample is estimated by comparing an observation result withsuch an electron beam diffraction pattern.

In the case of FIG. 14A, diffraction spots corresponding to the <110>incidence shown in FIG. 14C appear relatively clearly and hence it isconfirmed that the crystal axis is the <110> axis (the crystal surfaceis the {110} plane).

Respective spots have slight expanses on concentric circles, which isconsidered due to a certain distribution of a rotation angle about thecrystal axis. From the pattern, the degree of expanse is estimated to bewithin 5°.

Among a large number of observed patterns, there are patterns thatinclude a portion where no diffraction spot exists (the pattern of FIG.14A has a portion where no diffraction spot is found). This isconsidered due to the fact that the sample approximately has the {110}orientation but the crystal axis has a slight deviation.

Based on the fact that crystal surfaces almost always include the {111}plane, the inventors infer that a deviation in the rotation angle aboutthe <111> axis is a cause of the above phenomenon.

On the other hand, in the case of the electron beam diffraction patternof FIG. 14B, diffraction spots have no clear regularity and hence it isconfirmed that crystals are oriented almost randomly. That is, it isinferred that crystals having face orientations other than the {110}orientation are mixed irregularly.

As indicated by the above observation results, in a crystalline siliconfilm formed by the manufacturing method of the first embodiment, almostall crystal grains approximately have the {110} orientation and crystalshave continuity at grain boundaries. Conventional polysilicon films donot have this feature.

As described above, a semiconductor thin film formed by themanufacturing method of the first embodiment has an entirely differentcrystal structure (more correctly, crystal grain structure) fromconventional semiconductor thin films. The inventors described resultsof analyses on a semiconductor thin film to be used in the invention inJapanese Patent Application Serial Nos. Hei. 9-55633, Hei. 9-165216, andHei. 9-212428.

The inventors calculated orientation ratios of a crystalline siliconfilm formed by the above manufacturing method by performing X-raydiffraction according to the method described in Japanese PatentLaid-Open No. Hei. 7-321339. In this publication, orientation ratios aredefined by the following calculation formulae:

{220} orientation existence ratio)=1 (constant)

({111} orientation existence ratio)=(sample relative intensity of {111}to {220})/{powder relative intensity of {111} to {220})

({311} orientation existence ratio)=(sample relative intensity of {311}to {220})/{powder relative intensity of {311} to {220})

({220} orientation ratio)=({220} orientation existence ratio)/[({220}orientation existence ratio)+({111} orientation existence ratio)+({311}orientation existence ratio)]

FIG. 17 shows an example of a result of an X-ray diffraction measurementthat was conducted to determine an orientation of a semiconductor thinfilm formed by the above manufacturing method. The X-ray diffractionpattern has a peak corresponding to the (220) plane which is naturallyequivalent to the {110} plane. This measurement result showed that themain orientation was the {110} orientation and the orientation ratio was0.7 or more (typically 0.9 or more).

As described above, it is understood that the crystalline silicon filmformed by the manufacturing method of the first embodiment has anentirely different crystal structure from conventional polysiliconfilms. Based on this fact, it can be said that the crystalline siliconfilm of the invention is an entirely new semiconductor film.

In forming a semiconductor thin film according to the first embodiment,the annealing step that is executed at a temperature higher than thecrystallization temperature plays an important role for reduction ofdefects in grain boundaries. This will be described below.

FIG. 15A is a TEM photograph, taken at a magnification factor of250,000, of a crystal silicon film obtained at the stage that the stepsto the crystallization step of the manufacturing method of the firstembodiment have been finished. Zigzagged defects indicated by arrowsexist in a crystal grain (black and white portions appear due to adifference in contrast).

This type of defects are mainly stacking faults in which the stackingorder of atoms in silicon lattice planes are incorrect, and may also bedislocations or the like. The defects shown in FIG. 15A seem to bestacking faults having fault planes that are parallel with the {111}plane. This is inferred from the fact that the zigzagged defects arebent at an angle of about 70°.

On the other hand, as seen from a photograph of FIG. 15B that was takenat the same magnification factor, a crystal silicon film formed by themanufacturing method of the first embodiment has no defects such asstacking faults, dislocations, or the like in a crystal grain. It isconfirmed that the crystallinity is very high. This tendency holds forthe entire film area. Although it is currently difficult to make thenumber of defects zero, it is possible to reduce the number to a levelthat can be regarded as substantially zero.

That is, the crystal silicon film shown in FIG. 15B can be regarded orsubstantially regarded as a single crystal because the number of defectsin a crystal grain is reduced to a negligible level and grain boundariesnever act as barriers to carrier movement by virtue of their high levelof continuity.

As described above, although the crystal silicon films shown in thephotographs of FIGS. 15A and 15B have approximately equal levels ofcontinuity at grain boundaries, they are much different in the number ofdefects in a crystal grain. The fact that the electrical characteristicsof the crystal silicon film formed by the manufacturing method of thefirst embodiment shown in FIG. 11B are much superior to those of thecrystal silicon film shown in FIG. 15A is largely due to the differencein the number of defects.

The crystal silicon film (see FIG. 15B) formed by the manufacturingmethod of the first embodiment has the feature that the number ofdefects in a crystal drain is much smaller than the crystal silicon film(see FIG. 15A) that has been merely crystallized.

The difference in the number of defects appears as a difference in spindensity in an electron spin resonance (ESH) analysis. It has becomeapparent that at present the crystal silicon film formed by themanufacturing method of the first embodiment has a spin density of atmost 5×10¹⁷ spins/cm³ or less (preferably 3×10¹⁷ spins/cm³ or less).However, since this measurement value is close to the detection limit ofa currently available measurement instrument, it is inferred that theactual spin density is smaller than the above value.

The inventors call a crystal silicon film having the above crystalstructure and features continuous grain boundary crystal silicon (orcontinuous grain silicon: CGS).

While in conventional semiconductor thin films grain boundaries act asbarriers that obstruct movement of carriers, the semiconductor thin filmformed by the manufacturing method of the first embodiment realizes highcarrier mobility because substantially no such grain boundaries exist.Therefore, a TFT manufactured by using a semiconductor thin film formedby the manufacturing method of the first embodiment exhibit muchsuperior electrical characteristics. This will be described below.

Knowledge Relating to Electrical Characteristics of TFT

A semiconductor thin film formed by the manufacturing method of thefirst embodiment can substantially be regarded as a single crystalbecause substantially no grain boundaries exist. Therefore, a TFT usingit as an active layer exhibits electrical characteristics that areequivalent to those of a MOSFET using single crystal silicon. Thefollowing data have been obtained from TFTs that were experimentallymanufactured by the inventors.

(1) The subthreshold coefficient, which is an index of the switchingcharacteristic (quickness of on/off switching) of a TET, is as small as60-100 mV/decade (typically 60-85 mV/decade) in each of n-channel andp-channel TFTs.

(2) The field-effect mobility μ_(FE), which is an index of the operationspeed of a TFT, is as large as 200-650 cm²/Vs (typically 250-300 cm²/Vs)in an n-channel TFT and as 100-300 cm²/Vs (typically 150-200 cm²/Vs) ina p-channel TFT.

(3) The threshold voltage V_(th), which is an index of the drive voltageof a TFT, is as small as −0.5 to 1.5 V in an n-channel TFT and as −1.5to 0.5 V in a p-channel TFT.

As described above, it has been confirmed that a TFT that is muchsuperior in both switching characteristic and high-speed operationcharacteristic can be realized.

From the above discussions, it is understood that the catalyst elementgettering step is a step that is indispensable for formation of CGS. Theinventors assume the following model for a phenomenon that occurs inthis step.

In the state of FIG. 15A, the catalyst element (typically nickel) issegregated at defects (mainly stacking faults) in a crystal grain; thatis, there are many Si—Ni—Si bonds.

However, if Ni is removed from defects by executing the catalyst elementgettering process, Si—Ni bonds are disconnected. Resulting excessunconnected bonds of Si atoms immediately form new Si—Si bonds. Thedefects disappear in this manner and the bonding states are renderedstable.

Although naturally it is known that defects in a crystal silicon filmdisappear when thermal annealing is performed at a high temperature, themodel being discussed is different from this phenomenon. The inventorsinfer that re-combining of Si atoms occurs smoothly due to manyunconnected bonds that are produced by disconnection of Si—Ni—Si bonds.

The inventors also assume a model that the heat treatment at atemperature (700°-1,100° C.) higher than the crystallization temperaturecauses defects to disappear because a crystal silicon film is fixed tothe underlying member and the adhesion is increased.

Knowledge Relating to TFT Characteristics and CGS

The above-described superior TFT characteristics is largely due to theuse, as a TFT active layer, of a semiconductor thin film in whichcrystal lattices have continuity at grain boundaries. The reason will bedescribed below.

The continuity of crystal lattices at grain boundaries results from thefact that the grain boundaries are of a type called the “planarboundary.” The definition of the term “planar boundary” as used in thisspecification is the same as that of the “planar boundary” that isdescribed in Ryuichi Shimokawa and Yutaka Hayashi, “Characterization ofHigh-Efficiency Cast-Si Solar Cell Wafers by MBIC Measurement,” JapaneseJournal of Applied Physics, Vol. 27, No. 5, pp. 751-758, 1988.

According to this paper, the planar boundary includes the {111} twinboundary, the {111} stacking fault, the {221} twin boundary, the {221}twist boundary, etc. The planar boundary has a feature that it iselectrically inactive. That is, although the planar boundary is a grainboundary, it does not act as a trap that obstructs movement of carriers.Therefore, the planar boundary can be regarded as substantially absent.

In particular, the {111} twin boundary is called a coincidence grainboundary of Σ3 and the {221} twin boundary is called a coincidence grainboundary of Σ9. The Σ value is a parameter as an index that indicatesthe degree of matching of a coincidence grain boundary. It is known thata grain boundary having a smaller Σ value is higher in the degree ofmatching.

The inventors' detailed TEM observation of semiconductor thin filmsformed by the manufacturing method of the first embodiment has revealedthat most of (90% or more, typically 95% or more) of grain boundariesare coincidence grain boundaries of Σ3, that is, {111} twin boundaries.

It is known that a grain boundary formed between two crystal grains is acoincidence grain boundary of Σ3 if both crystal grains have a {110}orientation and θ=70.5° where θ is an angle formed by a lattice fringecorresponding to the {111} plane.

In the grain boundary in the TEM photograph of FIG. 13A, the latticefringe stripes of the adjacent crystal grains extend continuously toform an angle of about 70°. Therefore, it is easily presumed that thisgrain boundary is a {111} twin boundary.

If θ=38.9°, the grain boundary is a coincidence grain boundary of Σ9.Like this type of boundary, grain boundaries other than the {111} twinboundary also existed.

These types of coincidence grain boundaries are formed only betweencrystal grains having the same face orientation. That is, this type ofcoincidence grain boundaries can be formed over a wide area in asemiconductor thin film formed by the manufacturing method of the firstembodiment because face orientations are approximately the same ({110}orientation). Polysilicon films having irregular face orientations neverhave such a feature.

FIG. 16A is a TEM photograph (dark field image), taken at amagnification factor of 15,000, of a semiconductor thin film formed bythe manufacturing method of the first embodiment. There are regions thatlook white and regions that look black. The regions of the same colorhave the same orientation.

A feature in FIG. 16A that deserves special mention is that the whiteportions collectively exist. This means that crystal grains having thesame orientation exist with a certain degree of directivity and adjacentcrystal grains have almost the same orientations.

On the other hand, FIG. 16B is a TEM photograph (dark field image),taken at a magnification factor of 15,000, of a conventionalhigh-temperature polysilicon film. In this high-temperature polysiliconfilm, portions having the same face orientation are dispersed randomly;no collective structure having directivity like the one shown in FIG.16A is found. This is considered due to the fact that adjacent crystalgrains are oriented entirely irregularly.

The inventors repeated observations and measurements on thesemiconductor thin film of FIG. 13A at many regions other than themeasurement point of FIG. 13A, and have confirmed that the continuity ofcrystal lattices is secured in grain boundaries over a sufficientlylarge area to manufacture a TFT.

FIG. 18 is a TEM photograph of a light field image of a semiconductorfilm formed according to the manufacturing method of the thirdembodiment in which phosphorus is used in the nickel gettering step.FIGS. 19A and 19B are photographs of point 1 indicated in FIG. 18 atmagnification factors of three hundred thousand and two million,respectively. The region that is enclosed by a square in FIG. 19Acorresponds to FIG. 19B. FIG. 19C is an electron beam diffractionpattern (spot diameter: 1.7 μm) obtained at point 1.

Point 2 and point 3 were also observed under exactly the same conditionsas point 1. FIGS. 20A-20C show observation results of point 2 and FIGS.21A-21C show observation results of point 3.

It is seen from the above observation results that the continuity ofcrystal lattices is secured and a planar boundary is formed at any grainboundary. The inventors repeated observations and measurements on theabove semiconductor thin film at many regions other than the abovemeasurement points, and have confirmed that the continuity of crystallattices is secured in grain boundaries over a sufficiently large areato manufacture a TFT.

According to the invention, a nonvolatile memory can be formed on thesame substrate so as to be integral with its peripheral circuits such asa driver circuit and hence miniaturization is attained.

According to the invention, semiconductor active layers of a nonvolatilememory are relatively thin. Therefore, a nonvolatile memory in whichimpact ionization easily occurs and which can be driven at a low voltageand is less prone to deteriorate can be realized.

Further, since a nonvolatile memory according to the invention can beformed so as to be integral with parts of a semiconductor device, thesemiconductor device can be miniaturized.

1. A nonvolatile memory comprising: a plurality of memory cells over asubstrate, arranged in a matrix form, each of the memory cells includinga memory transistor portion and a switching transistor portion; whereinthe memory transistor portion includes a first semiconductor activelayer, a floating gate insulating film, a floating gate electrode, acontrol gate insulating film, and a control gate electrode, wherein theswitching transistor portion includes a second semiconductor activelayer thicker than the first semiconductor active layer, a switchinggate insulating film and a switching gate electrode, and wherein anaverage of a height difference between a convex and a concave on thesurface of the substrate is equal to or less than 5 nm.
 2. Thenonvolatile memory according to claim 1, wherein a thickness of thefirst semiconductor active layer and a thickness of the secondsemiconductor active layer are less than 150 nm.
 3. The nonvolatilememory according to claim 2, wherein a thickness of the firstsemiconductor active layer is within a range from 1 nm to 50 nm, andwherein a thickness of the second semiconductor active layer is within arange from 40 nm to 100 nm.
 4. The nonvolatile memory according to claim1, the substrate is a quartz substrate.